Danny Williams' last signing before leaving for Sheffield Wednesday, Horsfield joined the club from Newcastle United in May 1969, for a club record £17,500, for Swindon's first season in Division Two after their cup winning and promotion season in 1968/1969.

Horsfield made his debut on the opening day of the season in a 2-0 defeat at Blackburn on 9th August. He scored his first goal for the Town in his next game, an equaliser at Cardiff, and then he immediately endeared himself to the Town fans on his home debut, with two goals in the last twenty minutes to rescue a draw with the Town 2-0 down versus Carlisle.

He ended the season as the club's top scorer, with 22 goals, eighteen of which were in the league. He was also instrumental in the two European successes, first scoring a hat-trick in the home leg of the Anglo-Italian League Cup Winners Cup against Roma, and then the second goal in the Anglo-Italian Cup final at Napoli.

"King Arthur" to his fans, "Carthorse" to his critics, he stayed at Swindon until the end of the 1971/1972 season, when he moved to Charlton Athletic in an exchange deal, having scored 42 league goals in 108 appearances for the club.